1. Field of the Invention
The present invention relates to decorticated and fibrillated bast fibers as reinforcement for polymeric, thermoplastic, and thermoset composites.
2. Description of the Related Art
A polymer matrix composite (PMC) is defined as a matrix of plastic resins reinforced by fibers or other reinforcements with a discernible aspect ratio of length to diameter. Materials used to reinforce resins to provide superior strength, stiffness, impact resistance relative to weight include primarily glass, carbon, boron, aramids and cellulosic, or organic fibers. The fibrous reinforcements with a relatively high aspect ratios are distinctly different from fillers which are primarily in particulate or powdered form. Fillers for plastics include calcium carbonate, talc, mica, wollastonite, fly ash and other inorganic or organic compounds. The polymers may be either thermoset or thermoplastic resins and include polyesters, vinyl esters, epoxies, polyvinylchloride (PVC) and polyolefins such as polypropylene (PP) and low/medium/ high density polyethylene (LDPE, MDPE, HDPE).
The superior properties of the reinforced plastics makes them particularly useful for load bearing and structural applications. Polyolefins currently account for approximately 11.92 billion pounds of material, over 51% of the potential market. The total global annual consumption of reinforced plastics surpassed 23 billion pounds in 1999, and continues to grow at an overall rate of 5.4% per year. While both continuous and short fibers are used as reinforcement, a particular need is evident for the use of short lignocellulosic bast fibers such as flax, kenaf, jute, ramie, sisal, and hemp.
Natural bast fibers such as hemp, jute, flax, kenaf and sisal, have been used for tens of thousands of years to make paper, textiles, cordage and other products essential to human existence. Recently, there has been a resurgent interest in utilizing agricultural products as feedstock for industrial application. This trend is driven by several key factors, among them: 1) Reduction of dependence upon forest products and foreign petroleum; 2) Need to find alternatives to farm subsidies to support rural communities; 3) Elimination of air pollution caused by burning waste straw; 4) Desire to utilize more sustainable, less toxic natural resources.
In 1996, German environmental legislation mandated that cars must be able to be recycled. While the European automobile manufacturers found that they could successfully recover and recycle steel and rubber materials, they could do little with the glass fiber reinforced plastics used throughout vehicle interiors.
By combining natural fibers with polypropylene fibers in to non-woven mat products, then heating and pressing these mats into three-dimensional shapes, manufacturers could effectively produce interior trim components such as door panels, seat backs, package trays and instrument panels. Automobile manufacturers found that these natural fiber composites achieved a number of important benefits, including improved impact strength, significant weight reduction, lower manufacturing costs, greater dimensional stability, better acoustical performance, reduced waste generation, ability to recycle products, and safer work environments.
Flax (Linum usitatissimum L.) is grown as a commercial crop in Canada and the U.S. and harvested primarily for oil seed. Flax oil yields high quality solvents and lubricants such as linseed oil, and building materials such as linoleum flooring. Once harvested, the flax stalk becomes waste field straw. Because this straw cannot typically be plowed under, it poses a significant waste management problem for growers. The traditional disposal method is to burn it in the field, but this practice generates significant environmental and human health problems. Every 100,000 acres of flax straw burned produces the equivalent annual emissions of approximately 43,000 cars, or over 2 million pounds of green house gasses.
The traditional process for preparing the straw for reinforcement involves decortication. During decortication, the xe2x80x98shivexe2x80x99 core from the plant is removed and the fibers from the xe2x80x98barkxe2x80x99 of the plant is extracted. These long fibers, typically 4 to 6 inches in length are then used to prepare a non-woven, or needle punched mat with other polymeric fibers for use in compression molded parts.
For many years there has been a significant effort in research laboratories in North America, Europe and Asia to develop process technologies to effectively exploit the reinforcement properties of bast fibers in plastics. While a number of technologies have worked on a laboratory scale, the only commercial application of bast fibers has been in non-woven mats in compression molded automotive parts as described above. Since compression molding constitutes only about 20% of the installed base, the focus of research efforts has been to develop other methods to address a much larger market sector.
Every attempt in the past has resulted in problems similar to that quoted in U.S. Pat. No. 6,114,416 where the bast decorticated fiber due to its low bulk density xe2x80x98balls upxe2x80x99. Other terms used to describe the phenomenon is xe2x80x98clumpingxe2x80x99, xe2x80x98mattingxe2x80x99 or xe2x80x98hanging togetherxe2x80x99 of the fibers during compounding. The result of this has been an uneven and inconsistent distribution of the fiber in the resin matrix in the final product with areas that are resin rich and those that resin starved (fiber rich). Also, the surface finish of the parts is not smooth due to the effect of xe2x80x98clumping. Additionally, as reported in U.S. Pat. No. 6,114,416, the percent of bast fiber by weight that may be added to the resin is also very limited, typically much less than 10% by weight beyond which compounding, and molding of the composite specimen is not possible.
A prior method of fibrillation of bast fibers includes steam explosion. The STEX (steam explosion) process uses hydrolysis at elevated temperatures as its main method of removing unwanted constituents of flax, especially pectins, hemicellulose, and lignin. The processes described in the technical literature generally soak the flax with aqueous solutions prior to steam explosion. The thoroughly wet flax has adherent water, the acidity of which has been adjusted to the alkaline side in an attempt to reduce the degradation of the cellulose. A typical successful STEX process exposes the flax to 200 C. temperatures for 10 to 20 minutes. After quick release of the pressure, the steam-exploded flax usually is washed with an alkaline solution.
The effect of this procedure leads to a product that is high in cellulose percentage because most of the other polymers have been removed. Nevertheless, the composition of the cellulose has changed due to partial hydrolysis of this glucose polymer. The key indication of this damage is the degree of polymerization (DP) of the cellulose. Flax cellulose has DP of 1000 to 2000 glucose units. The reduction in DP depends on the severity of the conditions under which the STEX takes place. If the severity exceeds 3.0, the degradation is so drastic that the product is worthless. The most sophisticated STEX processes have a severity of about 2.7 which still provides a strong, useful product. Nevertheless, about 20% to 50% of the DP will be lost to associated hydrolytic action.
Through a combination of special processes, decorticated bast fibers are converted to a unique fibrillated state of matter, herein termed Fibex (FIBEX(trademark)), that overcomes all the difficulties in the stated above during compounding and molding. Fibex fibrillated bast fibers, also have superior characteristics over prior bast fibers.
While waste flax straw has been used as for demonstration of the invention, the general methodology covers all bast fiber materials listed above including flax, hemp, kenaf, jute, sisal, ramie, and similar bast fibers and lignocellulosic fibers. FIG. 1 shows the relative strengths of various pure bast fibers. As shown, flax is one of the strongest of the natural fibers.
The invention, in one form, comprises a fibrillated bast fiber composition including a decorticated bast fibers of which at least approximately 90% of the fibers have a cross-sectional area of less than approximately 700 micrometers squared.
The invention, in another, comprises form a fibrillated bast fiber composition including decorticated bast fibers which have been fibrillated without auto-hydrolysis, such that the fibrillated fibers have a molecular weight at least 75% of the molecular weight of the pre-fibrillated decorticated fibers.
The invention, in yet another form, comprises a fibrillated bast fiber composition including decorticated bast fibers which have been fibrillated without auto-hydrolysis such that the fibrillated fibers have a molecular weight at least 90% of the molecular weight of the pre-fibrillated decorticated fibers.
The invention, in still another form, is a process of forming a bast fiber composition comprising providing decorticated bast fibers; fibrillating the decorticated bast fibers utilizing mechanical impact; and admixing the fibrillated bast fibers with a polymeric resin. The fibrillating step, in the preferred form comprises the application of ultrasonic energy to the decorticated bast fibers. The application of ultrasonic energy in one form of the invention is conducted through liquid to said decorticated bast fibers.
One advantage of the present invention is having a much finer fiber that conventional decorticated materials as seen the scanning electron micrographs, FIGS. 2a and 2b. 
Another advantage of the present invention is that significantly greater surface area for bonding with the resin for the same quantity as compared to decorticated materials, as shown in FIG. 4.
Another advantage of the present invention is that effective wetting and dispersion during compounding with polymeric resins up to 50% by weight loading in the polymers.
Yet another advantage of the present invention is the availability, due to the prevention of clumping, of development of standard compounded pellets for ease of storage, transportation and handling.
Still another advantage of the present invention is the injection molding of specimens without any problems with xe2x80x98clumpingxe2x80x99 or balling up of fibers. Further, injection molding of parts using conventional equipment may be utilized and is now possible.
Yet another advantage of the present invention is a significant increases in stiffness and strength of the formed composites. Significant benefits in strength and stiffness to weight ratios approaching those of glass reinforced materials have been shown.